As others have pretty much indicated, it is the time, not the distance that seems to be the big limiting factor. On nice long summer days I often find myself extending my commute... just because I have the daylight. An extra 20-30 minutes doesn't matter if it is comfortable.

Back in my college days I had lots of early morning time and I would ride way way out of my way to get a good bike ride in before I hit class, then after class I would go to work (about a mile away) and then after work I would go home, (about 5 miles away), but the total mileage for the day would be about 45 miles, due to the long trip in.

Having an asymmetrical ride like that was actually fun, as I could really put in the miles when I felt like it.